WD has a habit of fiddling with their platters and heads.
The following benchmarks appear to show drives with two 500GB platters and 4 heads:
http://www.3dnews.ru/assets/external/illustrations/2010/04/29/591174/sata-r.jpg
http://s05.radikal.ru/i178/1009/14/ec15f950fa70.jpg
Your benchmark, OTOH, appears to be consistent with a drive having three 400GB platters and 5 heads. That would result in a reduction in the transfer rate by about 10%.
I’m only guessing, though. If you have access to a faster drive with the same model number, you might like to compare their weights.
